2.01.2011

Many of the challenges in ship building or ship maintenance stem from environmental concerns: from the seemingly minor in not wanting ship material or ship waste to enter to the ocean to the obvious preventing future spills or catastrophic failures like the Exxon Valdez. One of the solutions to making liquid-cargo ships more resistant to failure is a double hull design.

When major disasters like mining disasters or situations like the gulf oil spill occur one question people often ask is why we still have humans doing these dangerous jobs. And that comes into play with the welding necessary in assembling a double hull ship design. The space welders have to be in to do their jobs are very small with floors and girders blocking them in and the areas can get extremely hot during the day. All this combined means the work can be extremely dangerous to the people involved. Attempts have been made before to design welding robots that could do the work in place of human workers.

Welding carriages exist but these are limited by often being able only to operate within a few axes of motion. Larger and more flexible 6-axis machines exist but the overhead devices needed to hold these robots don't fit within the confines of a double hull structure. The authors of the study found a viable commercial option that is small and lightweight and able to operate within the small area required but still unable to make the important u-shaped welds often required in these spaces due to limited control and degrees of freedom of the robot.

The designers took the leading commercial welding robot design and came up with some additions to optimize it. One issue with getting the welding robot into the space is the small size of the access hole combined with a heavy and large robot can make it near impossible for human hands to guide it in. They developed a bridge plate that supports the weight of the robot and along with two winches would allow two workers to move the robot in and out of the access hole.

The final design uses both driving wheels on the floor along with passive wheels for guidance overcoming previous design weaknesses that relied on driving wheels only and could not move as accurately with dirt and debris on the ship floor. The rails it drives along fold up to allow it to move in and out of the closed space. It contains a six axis welding manipulator, comparable to other high end commercial devices, along with a sophisticated controller and positioning device. Six AC servo motors (encoders) are directed by a four layered logic modularized controller in the CPU.

Designers also reduced the size of the hand held controller needed to operate the robot as well as adding in the capability of it communicating wirelessly with the robot rather than via a cable. In the end they reduced the time required to weld, still met satisfactory welding standards, and based on field testing have some ideas for future design.

As in many other applications, some of the best developments come out of combining previous successful designs or seemingly slight improvements to the existing solution. This is also one case where field testing proved incredibly useful and I'm sure we'll see some interesting follow up from this study.

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